This page explains how to setup a statement that contains multiple accounts (in short: Multi-statement).
Go to Setup > Bank Reconciliation Setup > Multiple Statement Config > New
Create one record for all accounts you want to import using one file:
Enter a name
For Bank, select a file format or default bank configuration if applicable
Enter Folder ID, you may use the same one as for the bank statements
For CSV files only: Enter the number of the column that holds the identifier to the bank account (field: Identifier Column)
For CSV files only: Select the csv delimiter and number header and trailing rows if any
Save
Go to the Bank Account Configuration records needed to be imported this way (Setup > Bank Reconciliation Setup > Bank Account Configuration)
For each one, edit. Do three things:
For Bank, choose the default bank configuration (if applicable)
For Multiple Statement Record, chose the one created in step 2
For Multiple Statement Identifier, enter the identifier of the account
For CSV files, the value in this field should be equal to the value in the Identifier Column in the file
For MT940 files, use the account number (note that some MT940 formats include the currency at the end of the account number, i.e. NL99RABO0123456789EUR)
For CAMT.053 files, use the account number or IBAN
Remove the header and trailing rows if any (this must be set to zero)
Save
Now you are ready to import files.